[v8.5.7]docs: support logging session connect attrs to slow query log (#21468)

[ti-chi-bot]

This is an automated cherry-pick of #21468

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What is changed, added or deleted? (Required)

This PR updates docs according to pingcap/tidb#66617:

• Add Session_connect_attrs (json) to the schema examples of INFORMATION_SCHEMA.SLOW_QUERY and INFORMATION_SCHEMA.CLUSTER_SLOW_QUERY.
• Add the system variable doc for performance_schema_session_connect_attrs_size (default/range/-1 behavior/truncation/1 MiB hard limit note).
• Add status variable docs:
  • Performance_schema_session_connect_attrs_longest_seen
  • Performance_schema_session_connect_attrs_lost
• Update SESSION_CONNECT_ATTRS doc with size limit and truncation behavior.

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What is the related PR or file link(s)?

• This PR is translated from: docs: support logging session connect attrs to slow query log  docs#22547
  • *: support logging session connect attrs to slow query log tidb#66617
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[yibin87]

LGTM

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